Preparing planographic plates and solution therefor



Unite New York a No Drawing. Filed Mar. 18, 1957, Ser. No. 646,542

' 21 Claims. (Cl. 96-4) This invention relates to xerography and in particular to a xerographic method for preparing lithographic printing plates.

Lithographic printing, which is a type of planographic printing, is a well-known and established art. In general, the process involves printing from a flat (hence the term planographic) plate depending upon different properties of the image and non-image areas for printability. In lithography the non-image areas are hydrophilic while the image areas are hydrophobic. In the lithographic printing process, a fountain solution is applied to the plate surface which wets all portions of the surface not covered by the hydrophobic image. This solution keeps the plate moist and prevents it from scumrning up when the machine is stopped temporarily. An ink roll coated with a grease-based printing ink contacts the image surface depositing the lithographic ink only on the image areas-the hydrophilic non-image areas repelling the ink. The ink image may then be transferred directly to a paper sheet or other receptive surface but generally is transferred to a rubber offset blanket which in turn transfers the print to a final paper sheet. Hence, for each print made during a run, the planographic plate is first dampened with the aqueous fountain solution and then inked with a lithographic ink and finally printed.

The quality obtainable from planographic printing is quite high, approaching that of letter press printing. In addition, planographic plates may be used to obtain runs as long as one hundred thousand prints. However, the methods of placing images on planographic plates is generally a cumbersome and expensive procedure.

A planographic plate comprises either a zinc or aluminum sheet specially treated to make it water-receptive or, for shorter plate runs, comprises a paper base having high wet strength coated with a hydrophilic coating comprising a finely-divided pigment dispersed in a hydrophilic adhesive such as casein. Methods of placing images on such plates comprise typing using a special ribbon, writing on the plate with a grease pencil or utilizing a special photographically sensitized master whereby the image may be formed photographically.

More recently, a novel method known as xerography has come to the fore as a means of applying an image to a lithographic printing plate. The general xerographic process is described in U.S. 2,297,691 to Chester F. Carlson. As there described, the process, in general, comprises placing an electrostatic charge on a xerographic plate, exposing the plate to an image of light and shadow to be reproduced whereby the electrostatic charge is removed in those areas of the plate receiving light, thus producing an electrostatic charge pattern on the plate surface exactly corresponding to the original to be reproduced, contacting the plate bearing the electrostatic latent image with a finely-divided electroscopic marking material whereby the electrostatically charged marking particles deposit on the plate in image configuration. The powder image may then be transferred to any desirable image support base such as paper or, in the application described herein, to a lithographic plate.

The most generally used method of development in xerography is the cascade development method as described in 11.8, 2,618,552 to E. N. Wise.- However, other Patented Sept. 26, 1961 i7 cloud development, as described in US. 2,725,304 to Landrigan et al., or magnetic brush development wherein the finely-divided powder particles, called toner, are carried by iron filings (due to electrostatic attraction) which in turn are attracted magnetically by a permanent magnet. By merely brushing the surface of the plate with the clump of filings clinging to the end of the magnet, toner is removed by the electrostatic image which is thus developed or made visible.

The Xerographic plate is desirably sensitized by a corona discharge as described in US. 2,588,699 to C. F. Carlson. The xerographic plate generally used comprises a film of amorphous selenium on a conductive backing, generally aluminum. Such a plate is relatively expensive but may be used thousands of times without damage. Accordingly, the per-copy cost of the xerographic print is relatively low. However, the extra handling cost involved in, first, developing the powder image on the xerographic plate and then transferring and fixing the image on a suitable lithographic plate and then cleaning the xerographic plate for reuse represents time and therefore cost consuming operations. In addition, the resolution lost in the transfer step inherent in the process to gether with the necessity of storing and paying for both lithographic and xerographic plates are objectionable. Accordingly, it has long been desired to use the xerographic plate itself as the lithographic plate.

The lithographic plate, as described, consists primarily of a finely-divided pigment dispersed in a hydrophilic adhesive. One type of xerographic plate long known to the art has the same physical structure. Such a plate is described in US. 2,663,636 to A. E. Middleton and comprises a photoconductive pigment dispersed in an insulating binder. A well known photoconductive material readily available at low cost is zinc oxide and is described in US. 2,169,840 to Lewis et al. Moreover, zinc oxide is a widely used paper coating pigment. Unfortunately, the requirements placed on the adhesives in xerography are diametrically opposed to the needs of the lithographic art.

Most pigments useful in forming binder plates, such as zinc oxide, zinc sulfide, lead oxide, etc., have resistivities which are too low to hold an electrostatic charge in the dark. Accordingly, the insulating properties of the resin used in preparing binder plates are highly critical, particularly when the binder-pigment composition is coated on a paper backing. In general, it has been found that using a hydrophilic resin as the binder in a xerographic plate apparently is suflicient to form a thin surface film of water and consequently lower the surface resistance so that' there is considerable lateral leakage of the electrostatic charge. Such a plate is unable to hold an electrostatic latent image. Although xerographic binder plates have been available for many years, the only resins yet used commercially are silicone resins which are highly hydrophobic. Hence, before a xerographic binder plate can be utilized as a lithographic master, it is essential that the non-image areas of the plate be rendered hydrophilic.

A variety of means of accomplishing this have been investigated. One such method includes treating the nonimage areas with a mixture of zinc acetate, water and ethyl alcohol. The thought was that in such a treatment the alcohol would soften or remove the surface film of silicone resin while the hydrophilic zinc acetate would deposit on the exposed zinc oxide to render the surface hydrophilic in a manner analogous to the action of fountain solutions. Attempts to print with a plate so treated have uniformly been unsuccessful, it being seldom possi ble to obtain printing runs of more than a dozen copies without the non-image areas inking up. Other methods were investigated, but were also unsuccessful.

There have now been discovered processes. whereby excellent quality lithographic printing plates, suitable for long runs, may be prepared'directly from a xerographic binder-plate. In; generaL; the invention comprises; forrn ing a.binder.plate-of a photoconductive insulating, oxideor. sulfide and in particular of zinc. oxide,.zinc. sulfide,.

cadmium sulfide, lead oxide, or mixtures or1combina tions containing these photoconductive1 pigments either with each other or with other photoconductive-pigments such as mercuric sulfide, etc., in any resin having the requisite electrical properties for use in the xerographic process. A plate so prepared is'utilized in the regular xerographic process to form thereon a powder image.

which is permanently aifixed thereto as by heating.

The image-bearing xerographic plate is then immersed in a special treating solution for about 30 to 60 seconds. The treating solution comprises an aqueous solution of an acid at least as strong as acetic and hydrophilic anions forming insoluble compounds with ions of the photoconductive pigment. The hydrophilic anions may be added to the solution as a separate Water-soluble salt or may be provided by the acid itself as disclosed hereafter. Optionally, water-soluble dichromate or chromate may be added if desired. Plates so prepared have been utilized in printing runs of over 20,000 copies without appreciable deterioration of image quality in any respect.

The following examples are presented in illustration.

but not in limitation of the invention and it is to. be understood that the invention is to be limited only by the appended claims.

Example 1 A xerographic plate was prepared containing 2.5 parts of zinc oxide to one part of a silicone resin obtained from the General Electric Company, sold under the trade name. SR 82. The zinc oxide and silicone with enough toluene to give suitable grinding viscosity were ball milled to obtain a uniform dispersion of zinc oxide The resin-pigment mixture was in the resin solution. then coated on an 0.006-inch thick aluminum sheet and dried for three days. The plate was sensitized using a corona charging unit obtained from. The Haloid Company of.Rochester, New York, under the trade. name XeroX corona unit. The plate was then exposed to a line-copy image and developed using a magnetic brush and a xerographic toner comprising finely-dividedcgilr. The powder image was fixed, to the plate by.

sonite. heating for 90 seconds in an. oven at 290 F.

The plate. so prepared was then dipped in a treating solution consisting of 0.5% (by weight) of sodiumferrocyanide; 0.2% (by volume) of glacial acetic acid; and.

(by. Weight) of ammonium dichromatethe The plate was immersed for-.a

After removal from the treat- 0.0l balance being water. total time of 60 seconds.

ing solution the plate was rinsed with water and rubbed lightly with cotton dipped in a mixture of onevolume of Platex (manufactured by Addressograph-Multigraph Corporation of Cleveland, Ohio) and two volumes of water. The composition of Platex is not shown but it is believed to comprise a nickel salt, ammonium acid.

phosphate, butanal, cellulose gum, diethylene glycol, formaldehyde and water. on a lithographic press and wet with a fountain solution containing 0.1% (by weight) of sodium ferrocyanide;

0.05% (by volume) of glacial acetic acid; 1.0% (by.

weight) glycerine and the balance water. Six thousand prints were run off without deterioration of image quality.

Example 2 Example 1 was repeated excepting that the ammonium dichromate was omitted from the. treating solution.

Examination of the prints produced. by the lithographic, plates so prepared showed that, the image.dots,- particu-. larly the smaller dots, became waterreceptiveand did.

not print satisfactorily.

The plate was then mounted 4 Examples 3 and 4 Two lithographic plates were prepared and tested as in Example 1 except that in the treating solutions the amount of dichromate was 0.05% for Example 3 and 0.25% in Example 4. The image quality obtained. in Example 3 was equivalent to that in Example 1. In Example 4 it was found that the random powder par.- ticles present in the non-image areas became ink receptive during printing causing a deleterious amount of background in non-image areas.

Examples 5 through 12 A series of eight lithographic plates were prepared and tested as in Example 1 except that the following acids were used in the treating solution in place of acetic'acid: oxalic, tartaric, citric, tannic, hydrochloric, nitric, phosphoric and sulfuric acids. In each case the print quality was fully equivalent to that obtained in Example 1.

Examples 13 and 14 Lithographic plates were prepared and-tested as in- Example 1 except that in the treating solution for Example 13 potassium ferrocyanide was substituted'for sodium ferrocyanide and in Example 14 calcium ferrocyanide was substituted for sodium ferrocyanide. In each case the image quality of the prints obtained from these plates were fully equivalent to the image qualityobtained in Example 1.

Example 15 A lithographic plate was prepared and tested as in Example 1 except that potassium feiricyanide was used. in place of sodium ferrocyanide. Prints obtained from this plate were equivalent to those obtained in Example 1. However, the plate had to be kept in the dark as. under the influence of light zinc ferricyanide decomposes.

Examples 16 through 24 Examples 25 through 28 A series of four lithographic plates were prepared and tested as in Example 1 except that in the treating solution the concentration of'the acetic acid was varied'as follows: 0.5%; 1%; 2% and 4% respectively. In each case the prints obtained from the lithographic plates so prepared were fully equivalent in quality to those prepared 1 inExample 1.

Examples 29 through 35 Aseries of lithographic plates were prepared and tested as in Example 1 except that the ferrocyanide concentrations were as follows: 0.25%; 0.5%; 1%; 2%; 4%; 5% and 10%. In each case the immersion time. was one minute. In each case the prints so produced were fully equivalent to those produced in Example 1.

Example 36 A lithographic plate was prepared and tested as in: Example 1 except that aportion of the treating solution was usedas the fountain solution. The image. areas gradually became water receptive.

Example 37 A-- lithographic plate wasprepared. and tested..asin. Example l except that there was added to the fountain.

solution 0.002% ammonium dichromate. areas inked up slowly.

Examples 38 through 41 Four lithographic plates were prepared and tested as in Example 1 except that in the fountain solution the following concentrations of sodium ferrocyanide were used: 0.04%, 0.1%, 0.2% and 0.5%. The plates used with the fountain solution containing 0.1%, 0.2%, and 0.5 gave very good prints and long runs while the fountain solution containing only 0.04% sodium ferrocyanide was deemed inoperable.

Examples 42 and 43 Two lithographic plates were prepared and tested as in Example 1 except that the concentration of acetic acid in the fountain solution was increased to, 0.08% and 0.2%, respectively. Again excellent results were obtained with both plates.

Example 44 A xerographic plate was prepared containing 0.88 parts (by volume) of lead monoxide to 1 part (by volume) of dry Lucite 46 (a trade name of E. I. du Pont de Nemours & Company and believed to be a copolymer of isobutyl methacrylate and n-butyl methacrylate). The lead monoxide and methacrylate resin together with enough toluene to give suitable grinding viscosity were ball-milled to obtain an uniform dispersion of lead monoxide in the resin solution. The resin-pigment mixture was then coated on an 0.006-thick aluminum sheet and dried for three days. The plate so prepared was then immersed for 30 seconds in a treating solution consisting of 5% (by weight) of sodium ferrocyanide and 2% (by volume) of glacial acetic acid. After removal from the treating solution the plate was rinsed and then tested for water-receptivity. Whereas prior to immersion in the treating solution the plate had not been water-receptive, it was now found that the plate was highly water-receptive and easily wet.

The non-image Examples 45 through 47 Three xerographic plates were prepared as in Example 1 except that Lucite 46 was used as the binder instead of a silicone resin. Zinc oxide to resin ratio for each plate was 2.5 to 1, 5 to 1 and 7.5 to 1, respectively. None of the plates were found to be water-receptive as prepared. The plates were then immersed in a treating solution containing 5% (by weight) sodium ferrocyanide and 2% (by volume) of acetic acid. The plate having a ratio of 2.5 to 1 was tested after 30 seconds in the treating solution and found to be highly water-receptive. The plate having the zinc oxide to hinder ratio of 5 to 1 was found to be highly water-receptive when tested after seconds in the treating solution and the plate having the zinc oxide to binder ratio of 7.5 to 1 was found to be highly waterr'eceptive after only 5 seconds in the treating solution.

Example 48 A binder plate was prepared as in Example 44 excepting that in place of the lead monoxide there was used 12.6 parts (by weight) of zinc oxide and 18.4 parts (by weight) of mercuric sulfide to 5 parts (by weight) of the Lucite 46. The xerographic plate as prepared was found to be non-water-receptive. Upon immersion for 10 seconds in a treating solution containing 5% (by weight) of sodium ferrocyanide and 2% (by volume) of acetic acid, the plate was found to be highly water-receptive.

Example 49 A xerographic plate was prepared as in Example 44 excepting that the photoconductive pigment consisted of 17.6 parts (by weight) of zinc oxide and 14.4 parts (by weight) of lead monoxide to 5 parts (by weight) of Lucite 46. While the plate as initially prepared had no waterreceptivity, upon immersion for 10 seconds in a treating solution of 5%. (by weight) of sodium ferrocyanide and Example 50 A xerographic plate was prepared as in Example 1. The plate so prepared was then immersed for 60 seconds in a treating solution consisting of 0.4% (by Weight) of ammonium tungstate and 2% (by volume) of acetic acid. The plate so treated was found to be highly Waterreceptive.

Examples 51 and 52 Two plates were prepared and tested for water-receptivity as in Example 50 excepting that in place of ammonium tungstate the treating solutions contained respectively 0.4% (by weight) of ammonium molybdate and 5.0% of ammonium molybdate. In each case the plates were found to be highly water-receptive after this treatment.

Example 53 A xerographic plate was prepared and treated as in Example 50 excepting that in place of ammonium tungstate the treating solution contained 0.5% (by weight) of ammonium vanadate. The plate so treated was found to be highly water-receptive after immersion in the treating solution.

Example 54 containing respectively 5% (by weight) of oxalic, tannic and tartaric acids. The immersion time in the oxalic acid was 5 minutes; in the tartaric acid 4 minutes and in the tannic acid 2 minutes. In each case the plates so treated were found to be highly water-receptive. By contrast, a control immersed for 5 minutes in a 2% (by volume) solution of acetic acid was found to be still completely non-watenreceptive.

Example 58 A lithographic plate was prepared and tested as in Example 1 except that the zinc oxide coating contained 0.036% rose bengale dye. Prints obtained from the lithographic plate so prepared were fully equivalent to those obtained in Example 1.

In the process of the instant invention it is believed that the acid in the novel treating solutions reacts with the metallic oxide or sulfide of the photoconductive material to liberate ions therefrom. These ions then react with the hydrophilic anions contained in the treating solution to form an insoluble but hydrophilic compound on all exposed surfaces of the photoconductive pigment. In the case of the ferro-/or ferricyanides, the reaction may be either to form a simple metal ferro-/or ferricyanide or a complex ferro-/or ferricyanide as described, for example by A. E. Williams in his book Cyanogen Compounds, second edition, published by Edward Arnold Company, London, England, in 1948.

While a binder plate using any resin binder may be treated in acocrdance with the instant invention, the problem of rendering the binder plate hydrophilic, i.e., waterreceptive, is particularly acute when the binder is a silicone resin due to the intensely hydrophobic nature of these resins. For durability in long runs in the lithographic press, it is essential that the bond between the hydrophilic, insoluble compound and the rest of the pigment particles be as strong as possible. The strength of this bond appears to be a function both of the hydrophilic anions used in the treating solution and of the resin used in the xerographic plate. Because. of their outstanding performance in this respect, it is particularly. preferred.

in this invention to use ferrocyanides in the treating solution and a silicone resin as thebinder in the xerographic plate.

Immersion time in the treating solution is not at all critical and is dependent on the concentration of the ingredients in the treating solutions. Thus, for stronger treating solutions treating times of from to 10 seconds may be adequate While for weaker concentrations of treating solutions or less reactive anions, treating times of several minutes may be needed. Binder plates of the type used herein are well known to those skilled in the art and are described, for example, in US. patent application Ser. No. 311,546, filed September 25, 1952, by A. E. Middleton et al.

Although the reason is not known it has been observed that some xerographic plates when used in the process ofthe instant invention do. not print properly in the highlight areas. It has been found, however, that the addition of small amounts, as about 0.01 to 0.05% (by.

weight) of ammonium dichromate, improve the ink-receptive properties of the image dots, particularly in the highlight areas, without causing any random powder in nonimage areas to print. Many xerographic plates are apparently not subject to this difficulty and as to the remainder the use of a chromate or dichromate is necessary only if it is desired to obtain the quality possible.

While the present invention has been described herein.

insulating material comprising an electrically insulating resin hinder having suspended therein a finely divided photoconductive pigment selected from the group consisting of photoconductive insulating metal oxides and metal sulfides, said process comprising producing an electrostatic latent image on said photoconductive material, developing said electrostatic image with a hydrophobic developer powder, fixing said powder image to said material, and then treating the powder image-bearing material with an aqueous solution of an acid that reacts with said pigment to form cations of the metal thereof and said solution also containing anions forming a hydrophilic compound with the metal cations released from the pigment by the acid, said hydrophilic compound being insoluble in said aqueous solution.

2. A process according to claim 1 wherein said photoconductive pigment is zinc oxide.

3. A process according to claim 1 wherein said photoconductive pigment is tetragonal lead monoxide.

4. A process according to claim 1 wherein said anion is selected from the group consisting of ferrocyanide, ferricyanide, vanadate, molybdate, phosphate, tungstate, tartrate, tannate and oxalate.

5. A process according to claim 1 wherein said anion is ferrocyanide.

6. A process according to claim 1 wherein said photoconductive pigment is zinc oxide and said anion is ferrocyanide.

7. A process according to claim 1 wherein said photoconductive pigment is tetragonal lead monoxide and said anion is ferrocyanide.

8. A process for preparing a planographic printing plate from a xerographic plate, said xerographic plate comprising a relatively electrically conductive backing sheet coated on at least one side with a photoconductive insulating material comprising an electrically insulating resin. binder havingsuspended therein a-finely divided photoconductive pigment selected from the group consist- 8. ing of photoconductive insulating metal oxides and sulfides, said process comprising producing an electrostatic latent image on saidphotoconductivematerial, developing: said electrostatic image with a hydrophobic developer powder, fixing said powder image to said material, and then treating the powder image-bearing material with an aqueous solution of about 0.1% to about 0.5% (by weight) of a water-soluble ferrocyanide and an acid in a concentration to give said solution a pH equal to that caused by the addition of a solution of from about 0.1% to 2.0% (by volume) of glacial acetic acid.

9. A process according to claim 8 wherein said photoconductive pigment comprises predominantly zinc oxide.

10. A process according to claim 8 wherein said photoconductive pigment comprises predominantly tetragonal' lead monoxide.

11. Aprocess according to. claim 8 wherein said photo conductive pigment comprises predominantlyzinc sulfide.

12. A process according to claim 8 wherein said photoconductive pigment comprises predominantly cadmium sulfide.

13.- Aprocessaccording to claim 8 wherein said acid photoconductive pigment selected from the group consisting of photoconductive insulating metal oxides and sulfides, said process comprising producing an electrostatic latent image on said photoconductive material, developing said electrostatic image with a hydrophobic developer powder, fixing said powder image to said material,

and then treating the powder image-bearing material with an aqueous solution of from about 0.1% to about 5.0% (by weight) of a water soluble ferrocyanide, an acid in a concentration to give said solution a pH equal to that caused by theaddition of a solution of from about 0.1% to about 2.0% (by volume) of glacial acetic acid and about 0.001% to about 0.05% (by weight) ofa salt selected from the group consisting of water-soluble chromates and dichromates.

15. A process for preparing a planographic printing plate from a xerographic plate, said xerographic plate comprising. a relatively electrically conductive backing sheet coated on at least one side with a photoconductive insulating material comprising an electrically insulating resin binder having suspended therein a finely divided photoconductive pigment selected from the group consisting of photoconductive insulating metal oxides and metal. sulfides, said process comprising producing an electrostatic latent image on said photoconductive material, developing said electrostatic image with a hydrophobic developer powder, fixing said powder image to said material, and then treating the powder image-bearing material with an acidic aqueous solution of a watersoluble compound, said compound having a hydrophilic anion which forms water-insoluble compounds with metal cations of thephotoconductive metal oxides and metal sulfides, said solution having a low pH so as to react with said pigment to form cations of the metal thereof thereby causing said solution to react with said pigment to form a hydrophilic compound of said cation 1 of said water-soluble compound is a complex metal oxide anion.

17. A- process according to claim 15 whereinsaid .water-soluble compoundtis a. water-soluble phosphate.

18. A process for preparing a planographic printing plate from a xerographic plate, said xerographic plate comprising a relatively electrically conductive backing sheet coated on at least one side with a photoconductive insulating material comprising an electrically insulating resin binder having suspended therein finely divided photoconductive zinc oxide, said process comprising producing an electrostatic latent image on said photoconductive material, developing said electrostatic image with a hydrophobic developer powder, fixing said powder image to said material and then treating the powder imagebearing material with an acidic aqueous solution of a water-soluble compound, said compound having a hydrophilic anion which forms water-insoluble compounds with zinc cations, said solution having a low pH so as to react with said zinc oxide to form zinc ions thereby causing said solution to react with said zinc oxide to form a hydrophilic zinc compound on the free surfaces of said zinc oxide, said compound being insoluble in said solution.

19. A process according to claim 18 wherein the anion of said water-soluble compound is a complex metal oxide anion.

20. A process according to claim 18 wherein said watersoluble compound is a water-soluble phosphate.

21. The process according to claim 18 wherein said water-soluble compound is a water-soluble ferrocyanide.

References Cited in the file of this patent UNITED STATES PATENTS 1,599,147 Schmidt Sept. 7, 1926 2,093,421 Crowley Sept. 21, 1937 2,240,486 Beckley May 6, 1941 2,297,691 Carlson Oct. 6, 1942 2,323,799 Crowley July 6, 1943 2,333,221 Whyzmuzis Nov. 2, 1943 2,814,988 Bradstreet et al. Dec. 3, 1957 2,857,271 Sugarman Oct. 21, 1958 2,857,272 Greig Oct. 21, 1958 FOREIGN PATENTS 539,066 Great Britain Aug. 27, 1941 678,187 Great Britain Aug. 27, 1952 648,897 Great Britain Jan. 17, 1951 OTHER REFERENCES Sugarman: Electrofax-A New Tool for the Graphic Arts, R.C.A. (reprinted from The American Pressman, November 1955), pp. 1-6. 

1. A PROCESS FOR PREPARING A PLANOGRAPHIC PRINTING PLATE FROM A XEROGRAPHIC PLATE, SAID XEROGRAPHIC PLATE COMPRISING A RELATIVELY ELECTRICALLY CONDUCTIVE BACKING SHEET COATED ON AT LEAST ONE SIDE WITH A PHOTOCONDUCTIVE INSULATING MATERIAL COMPRISING AN ELECTRICALLY INSULATING RESIN BINDER HAVING SUSPENDED THEREIN A FINELY DIVIDED PHOTOCONDUCTIVE PIGMENT SELECTED FROM THE GROUP CONSISTING OF PHOTOCONDUCTIVE INSULATING METAL OXIDES AND METAL SULFIDES, SAID PROCESS COMPRISING PRODUCING AN ELECTROSTATIC LATENT IMAGE ON SAID PHOTOCONDUCTIVE MATERIAL, DEVELOPING SAID ELECTROSTACTIC IMAGE WITH A HYDROPHOBIC DEVELOPER POWDER, FIXING SAID POWDER IMAGE TO SAID MATERIAL, AND THEN TREATING THE POWDER IMAGE TO SAID MATERIAL, WITH AN AQUEOUS SOLUTION OF AN ACID THAT REACTS WITH SAID PIGMENT TO FORM CATIONS OF THE METAL THEREOF AND SAID SOLUTION ALOS CONTAINING ANIONS FORMING A HYDROPHILIC COMPOUND WITH THE METAL CATIONS RELEASED FROM THE PIGMENT BY THE ACID, SAID HYDROPHILIC COMPOUND BEING INSOLUBLE IN SAID AQUEOUS SOLUTION. 